This invention relates to an idling rpm feedback control method for internal combustion engines, and more particularly to a method of this kind which can prevent engine stall during transition of the engine operation from a decelerating condition with the throttle valve fully closed to the idling rpm feedback controlling condition.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded with electrical loads by head lamps, fans, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling rpm feedback control method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98628, which comprises setting desired idling rpm in dependence upon load on the engine, detecting the difference between the actual engine rpm and the desired idling rpm, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine rpm to the desired idling rpm.
According to this proposed method, if the idling rpm feedback control is carried out immediately when the engine is decelerated toward the desired idling rpm region with the throttle valve fully closed and before the engine speed drops to the desired idling rpm region, the resulting controlled quantity of supplementary air is much smaller than a required quantity to be supplied to the engine, because on such occasion the supplementary air quantity is controlled to such a small value as to immediately bring the engine rpm to the desired idling rpm. If on such occasion the clutch of the engine is disengaged, there occurs a sudden drop in the engine speed, which can cause engine stall.
To avoid this disadvantage, an idling rpm feedback control method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98629. According to this proposed method, in transition from a decelerating condition to an idling rpm feedback controlling condition, the quantity of supplementary air is controlled in decelerating mode wherein the quantity of supplementary air required for maintaining the engine rpm at the desired idling rpm is estimated in advance of the completion of the transition of the engine operation and the estimated quantity of supplementary air is previously supplied to the engine before the idling rpm feedback control is started, to thereby ensure smooth transition to the idling operation.
However, even if the above decelerating mode control of the supplementary air quantity is carried out in advance of the completion of the transition of the engine operation to the idling rpm feedback controlling condition, the engine speed can temporarily drop below the desired idling rpm before it is controlled to or in the vicinity of the desired idling rpm, if the rate of decrease in the engine rpm is large, for instance, when the clutch is disengaged while the engine is decelerating with large loads applied on it, such as electrical loads. If the rate of decrease in the engine rpm is too large, it will easily cause engine stall and give discomfort to the driver, as stated before.